The nature of the genetic mutations associated with AD strongly suggest that an abnormality in the expression or processing of the amyloid precursor protein (APP) leads to AD pathology. How might aging itself, which underlies the vast majority of AD cases, lead to a similar pathology? Recent in vitro evidence establishes that secreted fragments of APP, APPS, improve neuronal CA ++ homeostasis and promote neuronal survival; secreted ABeta has opposite effects. A disruption of this balance, between the neurotoxic ABeta and the neuroprotective APPS, now seems a compelling mechanism which might contribute to AD pathology. Homologous proteins, the amyloid precursor-like proteins (APLPs), likely have functional and pathophysiologic roles in APP biology as well. Using electrophysiologic recording techniques, fluorescent Ca++ -indicator dyes and neurotoxicity assays, we have designed experiments to examine how APPs and APLPs and APLPs modulate important processes involved in Ca++ homeostasis including: voltage-dependent calcium channels, robust synaptic activity, glutamate-receptor activation and Ca++ release from internal stores. Since APPs, ABeta, and APLPs are secreted, present in synaptic terminals, modulate intraneuronal [Ca++], and are neuroprotective in excitotoxicity models, we suspect they play an important role in synaptic transmission. To test this, we plan to examine the effects of APPs and APLPs in a model system that exhibits easily controlled synaptic activity. Known mechanisms of APP regulation in non-neural tissues combined with newly found functional properties, strongly suggest that the synthesis and secretion of the APP family of proteins are regulated by inherently neural processes. In this proposal we plan to carefully examine the effects of neurotransmitter-receptor activation, thermal and metabolic neural stress, and activated second messenger systems on APP, APPs, ABeta and APLP synthesis and secretion. Our preliminary data indicate that a variety of physiologically important conditions differentially affect APP, APPs and APLP levels in neurons and glia and their overlying media. Aging-related disorders, like genetic mutations, could alter the functional activity, processing or synthesis of these molecules contributing to AD neurodegeneration. To study effects due to aging itself, we will compare regulatory and functional properties of the APP family of proteins in mature, month old cultures with cultures grown for greater than 6 months. Our goal is to discover how the functional properties and regulation of the various members of the APP family of molecules are coordinated in normal and stressed CNS tissue. Our central hypothesis is that a disorder in this relationship could adversely affect neuronal Ca++ homeostasis and thereby contribute to neurodegeneration in AD.